The present invention relates in general to directional couplers which may be the stripline, microstrip, coaxial, MMIC or waveguide type, and more particularly to a directional coupler for providing an output signal and a coupled signal from an input signal while reducing the effects of a reflected signal.
In Radio Frequency (RF) systems, it is often desirable to determine the amplitude of a signal input into the system, a signal present in the system, or a signal output from the system. It is difficult to obtain a direct measurement of these signals; so, directional couplers are placed in RF systems at locations where a determination of the amplitude of a signal is desired. A directional coupler divides an input signal into a coupled signal and an output signal; the amplitude of the coupled signal being proportional to the amplitude of the input and output signals. The amplitude of the input and output signals can thus be determined by measuring the amplitude of the coupled signal. An accurate measurement of the amplitude of the coupled signal is important to an accurate determination of the amplitude of the input and output signals.
A reflected signal can occur at a discontinuity between the directional coupler and an external component (or load) attached to the directional coupler and can distort the coupled signal resulting in an inaccurate measurement of the amplitude of the coupled signal. Therefore, most systems attempt to match the impedance of the output port to the impedance of the load. The closer the impedance match, the lower the amplitude of the reflected signal. However, it is very difficult to match the impedance over an entire frequency band; therefore, typical design techniques match the impedance at a specific desired frequency, and the impedance match degrades as the frequency deviates from that frequency.
For many systems, the inaccuracy of the measurement of the amplitude of the coupled signal is inconsequential to system performance. However, it is anticipated that future systems will be significantly more sophisticated and require a more accurate determination of the amplitude of an input or output signal; inaccuracies of the measurement of the coupled signal induced by a reflected signal will not be acceptable. Therefore, it is desirable to have an improved directional coupler that reduces the portion of the reflected signal that adds to the coupled signal.